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This patch implements C++23 P2334R1, which is easy because Joseph has done
all the hard work for C2X already.
Unlike the C N2645 paper, the C++ P2334R1 contains one important addition
(but not in the normative text):
"While this is a new preprocessor feature and cannot be treated as a defect
report, implementations that support older versions of the standard are
encouraged to implement this feature in the older language modes as well
as C++23."
so there are different variants how to implement it.
One is ignoring that sentence and only implementing it
for -std=c++23/-std=gnu++23 like it is only implemented for -std=c2x.
Another option would be to implement it also in the older GNU modes but
not in the C/CXX modes (but it would be strange if we did that just for
C++ and not for C).
Yet another option is to enable it unconditionally.
And yet another option would be to enable it unconditionally but emit
a warning (or pedwarn) when it is seen.
Note, when it is enabled for the older language modes, as Joseph wrote
in the c11-elifdef-1.c testcase, it can result e.g. in rejecting previously
valid code:
#define A
#undef B
#if 0
#elifdef A
#error "#elifdef A applied"
#endif
#if 0
#elifndef B
#error "#elifndef B applied"
#endif
Note, seems clang went the enable it unconditionally in all standard
versions of both C and C++, no warnings or anything whatsoever, so
essentially treated it as a DR that changed behavior of e.g. the above code.
After feedback, this option enables #elifdef/#elifndef for -std=c2x
and -std=c++2{b,3} and enables it also for -std=gnu*, but for GNU modes
older than C2X or C++23 if -pedantic it emits a pedwarn on the directives
that either would be rejected in the corresponding -std=c* modes, e.g.
#if 1
#elifdef A // pedwarn if -pedantic
#endif
or when the directives would be silently accepted, but when they are
recognized it changes behavior, so e.g.
#define A
#if 0
#elifdef A // pedwarn if -pedantic
#define M 1
#endif
It won't pedwarn if the directives would be silently ignored and wouldn't
change anything, like:
#define A
#if 0
#elifndef A
#define M 1
#endif
or
#undef B
#if 0
#elifdef B
#define M 1
#endif
2021-10-06 Jakub Jelinek <jakub@redhat.com>
libcpp/
* init.c (lang_defaults): Implement P2334R1, enable elifdef for
-std=c++23 and -std=gnu++23.
* directives.c (_cpp_handle_directive): Support elifdef/elifndef if
either CPP_OPTION (pfile, elifdef) or !CPP_OPTION (pfile, std).
(do_elif): For older non-std modes if pedantic pedwarn about
#elifdef/#elifndef directives that change behavior.
gcc/testsuite/
* gcc.dg/cpp/gnu11-elifdef-1.c: New test.
* gcc.dg/cpp/gnu11-elifdef-2.c: New test.
* gcc.dg/cpp/gnu11-elifdef-3.c: New test.
* gcc.dg/cpp/gnu11-elifdef-4.c: New test.
* g++.dg/cpp/elifdef-1.C: New test.
* g++.dg/cpp/elifdef-2.C: New test.
* g++.dg/cpp/elifdef-3.C: New test.
* g++.dg/cpp/elifdef-4.C: New test.
* g++.dg/cpp/elifdef-5.C: New test.
* g++.dg/cpp/elifdef-6.C: New test.
* g++.dg/cpp/elifdef-7.C: New test.
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|---|---|---|
| .. | ||
| include | ||
| po | ||
| aclocal.m4 | ||
| ChangeLog | ||
| ChangeLog.jit | ||
| charset.c | ||
| config.in | ||
| configure | ||
| configure.ac | ||
| directives.c | ||
| errors.c | ||
| expr.c | ||
| files.c | ||
| generated_cpp_wcwidth.h | ||
| identifiers.c | ||
| init.c | ||
| internal.h | ||
| lex.c | ||
| line-map.c | ||
| location-example.txt | ||
| macro.c | ||
| Makefile.in | ||
| makeucnid.c | ||
| mkdeps.c | ||
| pch.c | ||
| symtab.c | ||
| system.h | ||
| traditional.c | ||
| ucnid.h | ||
| ucnid.tab | ||